Conventionally, lead-tin alloy (Pb—Sn alloy) solder is used for bonding a semiconductor substrate such as an IC chip having a semiconductor device to a base member such as a heat sink and a lead frame. However, in view of environmental protection such as safety and low harmfulness, it is required to use Pb free solder or low Pb contained solder. One of Pb free solders is a tin-based solder.
Japanese Patent Application Publication No. 2003-347487 discloses that a backside of a semiconductor substrate is bonded to a base member with a tin-based solder. The substrate includes a backside electrode. The electrode is composed of a titanium layer, a nickel layer and a gold or a silver layer, which are stacked on the substrate in this order. The base member is a heat radiation member. The substrate is mounted on the heat radiation member with the tin-base solder. The tin-based solder is made of gold and tin alloy so that the solder provides a low melting solder.
The above semiconductor device is manufactured as follows. Firstly, the backside of the substrate is polished and cleaned. Then, the backside electrode is formed on the polished surface of the substrate. Then, the tin-based solder is sandwiched between the heat radiation member and the backside electrode. Then, the tin-based solder is heated up to a solidus temperature of the solder. Thus, the solder is reflowed so that the substrate is soldered on the heat radiation member.
In this structure of the backside electrode, the Ti layer provides adhesiveness and ohmic contact of a semiconductor substrate made of silicon. The Ni layer provides bonding property of the solder by alloying the Ni layer and tin in the solder. Thus, the substrate and the base member are electrically, thermally and mechanically bonded with the solder.
Here, a passive film such as an oxide film is easily formed on the Ti layer, and it is difficult to remove. The passive film is formed in atmosphere having ordinary temperature and ordinary pressure. Therefore, it is difficult to directly bond the Ti layer to the solder. Thus, it is required to form the Ni layer on the Ti layer sufficiently thickly. Further, it is required to control a soldering condition with high accuracy. Therefore, a manufacturing cost becomes higher.